1. Field of the Invention
The present invention relates to an apparatus for reducing the concentration of carbon monoxide included in a carbon monoxide-containing hydrogen-rich gas and also to a method of the same.
2. Description of the Prior Art
Some proposed apparatuses for reducing the concentration of carbon monoxide use an Au/Fe.sub.2 O.sub.3 catalyst supported on alumina or another support (for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 7-185303 and 7-196302). When a hydrogen-rich gas and a predetermined amount of oxygen are fed into such an apparatus, the Au/Fe.sub.2 O.sub.3 catalyst accelerates the oxidation reaction of carbon monoxide preferentially over the oxidation reaction of hydrogen, thereby decreasing the concentration of carbon monoxide included in the hydrogen-rich gas.
These apparatuses for reducing the concentration of carbon monoxide are typically used in a fuel-cells system, for example, including polymer electrolyte fuel cells or phosphate fuel cells. The following shows electrochemical reactions occurring in such fuel cells: EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.- (1) EQU 2H.sup.+ +2e.sup.- +(1/2)O.sub.2.fwdarw.H.sub.2 O (2)
H.sub.2 +(1/2)O.sub.2.fwdarw.H.sub.2 O (3)
Equation (1) shows the reaction occurring on the anode of the fuel cells; Equation (2) the reaction occurring on the cathode of the fuel cells; and Equation (3) the reaction occurring in the whole fuel cells. As clearly understood from these equations, for the progress of the cell reactions in the fuel cells, it is required to feed a supply of a hydrogen-containing gaseous fuel to the anode and a supply of an oxygen-containing oxidizing gas to the cathode. In case that these gases are contaminated with carbon monoxide, carbon monoxide is adsorbed by a platinum catalyst incorporated in the fuel cells and thereby lowers the catalytic function of the platinum catalyst. The air is generally used as the oxidizing gas and does not contain a significant amount of carbon monoxide that lowers the catalytic function. The gaseous fuel is, on the other hand, generally contaminated with a small amount of carbon monoxide, which may interfere with the decomposition reaction of hydrogen proceeding on the anode and deteriorate the performance of the fuel cells.
The contamination of the gaseous fuel with carbon monoxide is ascribed to the mechanism of producing the gaseous fuel through the reforming reaction of a hydrocarbon. The fuel-cells system typically includes a specific fuel reformer, which reforms a hydrocarbon to a hydrogen-rich gaseous fuel and supplies the resulting gaseous fuel to the anode of the fuel cells. The following reaction of steam reforming methanol is an example of such reforming reactions: EQU CH.sub.3 OH.fwdarw.CO+2H.sub.2 (4) EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 (5) EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2 (6)
In the process of steam reforming methanol, the decomposition reaction expressed by Equation (4) proceeds simultaneously with the reforming reaction of carbon monoxide expressed by Equation (5). The reaction of Equation (6) accordingly proceeds as a whole and produces a carbon dioxide-containing hydrogen-rich gas. In case that these reactions are completely shifted to the right side, no carbon monoxide exists in the final stage. In the actual fuel reformer unit, however, it is impossible to shift the reaction of Equation (5) completely to the right side. A trace amount of carbon monoxide is thus included as a by-product in the gaseous fuel produced by the fuel reformer unit.
The carbon monoxide concentration reduction apparatus is accordingly used to reduce the concentration of carbon monoxide included in the gaseous fuel fed to the fuel cells. The following Equation (7) shows the oxidation reaction of carbon monoxide proceeding in the carbon monoxide concentration reduction apparatus. The allowable concentration of carbon monoxide in the gaseous fuel fed to the fuel cells is not greater than several percents in the case of phosphate fuel cells and not greater than several ppm in the case of polymer electrolyte fuel cells.
CO+(1/2)O.sub.2.fwdarw.CO.sub.2 (7)
The Au/Fe.sub.2 O.sub.3 catalyst accelerating the oxidation reaction of carbon monoxide has a narrow effective temperature range (60 to 80.degree. C.) that ensures a sufficient catalytic activity for reducing the concentration of carbon monoxide. At the temperatures lower than the effective temperature range, the low oxidation activity of the catalyst does not sufficiently accelerate the oxidation reaction of carbon monoxide, which results in insufficient reduction of the concentration of carbon monoxide. At the temperatures higher than the effective temperature range, on the other hand, a small amount of carbon monoxide existing in the gaseous fuel is not selectively oxidized. Under this condition, affluent hydrogen is oxidized, and the oxidation reaction of carbon monoxide is not sufficiently carried out.
In order to reduce the concentration of carbon monoxide sufficiently, a precise regulation of the inner temperature of the carbon monoxide concentration reduction apparatus to the above effective temperature range is required according to the flow rate of the reformed gas that is subjected to the selective oxidation reaction of carbon monoxide. Rare metals, such as platinum, palladium, and rhodium, other than the Au/Fe.sub.2 O.sub.3 catalyst are known as the CO selective oxidizing catalyst. These rare metals have wider effective temperature ranges than that of the Au/Fe.sub.2 O.sub.3 catalyst. In case that the fuel cells receiving a supply of the reformed gas containing a reduced concentration of carbon monoxide are used as a power source for driving a vehicle, the amount of the reformed gas to be processed by the carbon monoxide concentration reduction apparatus remarkably varies with a significant variation in loading. The catalyst having the wider effective temperature range that ensures a high catalytic activity for selective oxidation reaction of carbon monoxide facilitates the regulation of the inner temperature of the carbon monoxide concentration reduction apparatus to the effective temperature range.
In some cases, however, the concentration of carbon monoxide is not sufficiently reduced, even when the inner temperature of the carbon monoxide concentration reduction apparatus is kept within the effective temperature range. This is because the catalyst for accelerating the oxidation reaction of carbon monoxide also has an activity for accelerating the production of carbon monoxide. In the carbon monoxide concentration reduction apparatus, the reforming reaction of carbon monoxide expressed by Equation (5) (hereinafter referred to as the shift reaction) and a reverse reaction of Equation (5) (hereinafter referred to as the reverse shift reaction) proceed in addition to the oxidation reaction of carbon monoxide expressed by Equation (7). The reverse shift reaction produces carbon monoxide. The following Equation (8) shows the reverse shift reaction, that is, a reverse of the reforming reaction of carbon monoxide expressed by Equation (5). The shift reaction of Equation (5) is exothermic, whereas the reverse shift reaction of Equation (8) is endothermic. EQU H.sub.2 +CO.sub.2.fwdarw.H.sub.2 O+CO (8)
The reactions of Equations (5) and (8) are reversible. A variation in concentration of any one of the reactants and products or a variation in surrounding temperature shifts the equilibrium, and accelerates either the shift reaction of Equation (5) or the reverse shift reaction of Equation (8). In the effective temperature range that ensures a sufficient activity of the CO selective oxidizing catalyst for selective oxidation of carbon monoxide (for example, 100 to 160.degree. C. in the case of the platinum catalyst), the endothermic reverse shift reaction of Equation (8) proceeds to produce carbon monoxide.
When the oxidation reaction of carbon monoxide expressed by Equation (7) sufficiently proceeds, the degree of the reverse shift reaction expressed by Equation (8) is significantly smaller than the degree of the oxidation reaction of carbon monoxide expressed by Equation (7). Under such conditions, the concentration of carbon monoxide in the reformed gas is sufficiently reduced. In case that the oxidation reaction of carbon monoxide is concluded before the reformed gas fed to the carbon monoxide concentration reduction apparatus has passed through the surface of the CO selective oxidizing catalyst, however, only the reverse shift reaction of Equation (8) proceeds between the position of the conclusion of the oxidation reaction and the position of the discharge of the reformed gas. When it is required to reduce the concentration of carbon monoxide to the ppm level, the amount of carbon monoxide produced by the reverse shift reaction is negligible. The amount of oxygen introduced into the reformed gas for the oxidation reaction of carbon monoxide is generally determined according to the flow rate of the reformed gas fed to the carbon monoxide concentration reduction apparatus. Oxygen is accordingly used up at the time point when the oxidation reaction of carbon monoxide is concluded. The oxidation reaction of Equation (7) can thus not proceed to consume carbon monoxide produced by the reverse shift reaction proceeding after the conclusion of the oxidation reaction of carbon monoxide. The reformed gas containing carbon monoxide produced by the reverse shift reaction is according fed from the carbon monoxide concentration reduction apparatus to the fuel cells.
As discussed above, even when the inner temperature of the carbon monoxide concentration reduction apparatus is kept within the effective temperature range that ensures a high catalytic activity for selective oxidation of carbon monoxide, the conventional carbon monoxide concentration reduction apparatus may not sufficiently reduce the concentration of carbon monoxide due to carbon monoxide produced by the reverse shift reaction. Especially in case that a significant variation in loading connected to the fuel cells varies the amount of the reformed gas to be processed by the carbon monoxide concentration reduction apparatus, a change in space velocity in the apparatus worsens the problem of the reverse shift reaction. The space velocity represents a volume of the supplied gas per unit volume of the catalyst and unit hour and shown by the unit of h.sup.-1. The decrease in amount of the reformed gas to be processed by the carbon monoxide concentration reduction apparatus with a decrease in loading lowers the space velocity and causes the amount of the catalyst to be excess over the amount of the reformed gas subjected to the selective oxidation reaction of carbon monoxide. This quickly concludes the oxidation reaction of carbon monoxide and increases the amount of carbon monoxide produced by the reverse shift reaction. When an increase in loading enhances the space velocity in the carbon monoxide concentration reduction apparatus, on the other hand, the supply of the reformed gas is made excess over the processing ability of the catalyst. This results in insufficient selective oxidation of carbon monoxide and causes the reformed gas containing the non-oxidized residual carbon monoxide to be fed from the carbon monoxide concentration reduction apparatus to the fuel cells.